Content providers increasingly rely on content delivery networks (CDNs) to distribute content requested by a geographically diverse clientele. Content delivery networks reduce delivery costs, improve performance and robustness, and enhance a quality of experience for end users by caching copies of content in numerous locations to reduce the distance between end users and a given copy of the content.
In general, a CDN includes a distribution system made up of content serving nodes, or “surrogates,” that cache copies of content and deliver the content in response to requests from end user devices. The distribution system also distributes content within the CDN, with content typically originating at an origin server and being distributed among the surrogates that are closer, both topologically and geographically, to a service provider edge and therefore to the end users. A request router of the CDN functions as a request reference point for the CDN to end user devices. An end user device sends a content request (or “resource request”) to the request router, which selects a capable surrogate to handle the content request and then redirects the end user device to the selected surrogate. The end user device reissues the content request directly to the selected surrogate, which delivers the requested content to the end user device.
A content service provider (CSP), like a CDN provider, provides to end users a content service that includes delivery of content, identified by a content identifier, to the end users in response to content requests. In some cases, a CSP or CDN provider may elect to delegate content request handling for a subset of end users to another CDN, thereby creating a hierarchical relationship between the CSP or CDN provider that is delegating content delivery (the “upstream entity”) and the delegated, downstream CDN that then either selects a surrogate for content delivery or delegates content request handling for the subset of users to a still further downstream CDN.
In some instances, a CSP may be associated, through contractual or other arrangements, with a particular CDN that is responsible for delivering content in responsive to content requests directed to the content service of the CSP. This CDN, the “authoritative” or “primary” CDN of the CSP, may elect to delegate content request handling for subsets of end users to respective “secondary” CDNs. Secondary CDNs thus maintain a transitive relationship for delivery of CSP content via the primary CDN. When an end user device associated with a secondary CDN issues a content request to a CSP, the CSP redirects the content request to the primary CDN, which in turn refers the content request to the secondary CDN for still further redirection to a local media cache of the secondary CDN that stores a copy of the content.